The present invention relates to a spark plug having a noble metallic firing tip provided on the surface of a central electrode or a grounded electrode, preferably installed in an internal combustion engine.
To extend the life and also to improve the performance of the spark plug for an internal combustion engine, a Pt alloy is preferably used as a material for the central electrode and/or the grounded electrode which cooperatively cause an electrical discharge.
In view of the recent trends toward the exhaust gas purification and lean burn techniques, it is desirable to reduce the diameter and also to enlarge the axial length of each electrode to improve the ignitability.
However, the Ni alloy spark electrodes, when reduced in diameter and enlarged in axial length, are subjected to exhaustion or wear in use. This will undesirably increase the spark gap between the Ni alloy spark electrodes, worsening the ignitability significantly.
To solve this problem, it is effective to provide a noble metallic firing tip on at least one confronting surface of the central electrode and the grounded electrode.
The bonding of the noble metallic firing tip to the electrode member is, for example, performed by the laser welding.
Unexamined Japanese Patent Application No. Kokai 6-36856 discloses a spark plug fabricated by using the laser-welding.
According to this prior art, the spark plug comprises a base electrode member having a smaller-diameter rod portion. A noble metallic firing tip, identical in diameter with the smaller-diameter rod portion, is bonded to the top of the smaller-diameter rod portion by the laser welding. A fused junction layer, in which the base electrode member and the noble metallic firing tip are fused together and then hardened, is formed between the base electrode member and the noble metallic firing tip.
FIGS. 11A, 11B and 11C show the detailed bonding steps of the noble metallic firing tip to the electrode member. First, a noble metallic firing tip 3 is placed on a top surface 911 of a smaller-diameter rod portion 91 of a base electrode member 9 as shown in FIGS. 11A and 11B. Then, the noble metallic firing tip 3 and the base electrode member 9 are bonded by using a laser beam 5 applied entirely along their circumferential boundary 93 as shown in FIG. 11C. Through the laser welding performed in this manner, the noble metallic firing tip 3 and the base electrode member 9 are fused together. And then, after finishing the laser welding, the fused portion is hardened to leave a fused junction layer 4 having a wedge shape.
In this laser welding, the base electrode member 9 is subjected to the scattering of the base metal due to the sputtering. Accordingly, as shown in FIG. 11C, a necked portion 49 having a reduced diameter is formed near the bonding boundary of the base electrode member.
The above-described conventional spark plug has the following problems.
From the view point of the global environmental protection, the recent restrictions to the exhaust gas emission and to the fuel economy have been becoming severe. To respond to such severe requirements, the spark plugs must be highly advanced in performances to realize the lean burn or other advanced techniques. To this end, the noble metallic firing tip, provided at the electric discharge portion, needs to be small in diameter.
Under such circumstances, the phenomenon that the laser welding substantially reduces the radial size of the base electrode member 9 becomes a problem not negligible when the noble metallic firing tip 3 and the small-diameter rod portion 91 of the base electrode member 9 need to be downsized in their diameters. The following are the problems arising from the radial size reduction of the base electrode member 9 by the laser beam 5.
More specifically, the spark plug equipped in an internal combustion engine will be subjected to high temperatures when the internal combustion engine is driven at high speeds. In this case, the noble metallic firing tip 3 cannot smoothly transfer the received heat to the base electrode member 9 due to a narrowed bonding portion of the base electrode member 9 adjacent to the noble metallic firing tip 3. Accordingly, a remarkable temperature increase occurs in the noble metallic firing tip 3. This will induce the extraordinary wear at the electric discharge portion.
Furthermore, the small-diameter rod portion 91 is so weak against high temperatures that it thermally deforms and causes the noble metallic firing tip 3 to fall or drop out of the small-diameter rod portion 91.